klue wrote:Its all great in theory, and yes I agree you guys are technically right, and eloquently put at that .But Maybe this VC works on some sort of duty cycle, or frequency hard to explain but for instance:

launching my car under boost, spins the front passenger wheel consistently even though the car is accelerating. You can hear it squeel, but still peel

anyone else have a similar real time situation?

You are simply unloading all the power going to the front on that one wheel. You are still accelerating because approx 50% of power is going to the back, and the rear wheels are pushing you forward. A front LSD would fix this problem.

i just did a 6000 rpm launch the other day with co works watching and all 4 spon with smoke on all 4 wheels .yes the front will spin more at times that is normal.evos do the same .wrx will do more to the rear.wish i had video of the launch because it was tits!!!!!!

Luke, as RedCelicaTRD said; when launching your front right tire is experiencing a lack of traction and due to your front differential being open you are sending most of the 50% of the front axles torque to that wheel. The rear wheels are still receiving 50% of engine torque and you will continue to move forward because of this. With regard to esracing's launch he was able to break traction on all four wheels with half engine torque to the front and half to the rear. Even though all four wheels were slipping they were all gripping an equal amount and all received identical power.

I took my car to my friends shop tonight and we tried a few tests to see how the 4WD system reacted. Ideally we would have reproduced the test used in the ramp video but we didn't have such a nice ramp available to play with and opted instead to try and duplicate this test: http://www.youtube.com/watch?v=9B0ANOI3GcM using these positioning jacks and another test up on the lift.

Before commencing these tests my car sat outside in ~6°C weather for over 3 hours so everything was completely cold. Also to my knowledge my car is equipped with all stock differentials, open front, viscous center, open rear.

Firstly when the car was put on the lift and then put in first gear with all wheels receiving 0 traction; all 4 wheels moved in the same direction simultaneously. No detectable delay in the rears catching up. We tried the lift test with the e-brake on (by accident) and when dropping the car into gear only the front wheels moved but the engine was beginning to stall as the power being sent to the rears wasn't being used and therefore the bias on the viscous coupler caused engine braking.

We then propped the front wheels up on the jacks to simulate slipping across the entire front axle. Unfortunately the jacks acted more like an e-brake for the front wheels as they weren’t really designed for being driven on however when the wheels did get rolling the jacks would meander and the car would start to push them out, bringing the test to a halt.

Initially when put on the jacks and driven in reverse the car began driving backwards with only the rear wheels moving! The car acted in a similarly to when the e-brake was engaged up on the lift, it wanted to stall.Eventually with some more driving in reverse the car did manage to briefly spin the wheels on the jacks as well as the wheels in the rear however they spun at different velocities, the rear's were constantly turning whilst the fronts only got a quarter turn in before stopping again.

We then tried driving forward with the front wheels on the jacks and managed to get all four wheels moving. The rears were getting the power out much more easily with the fronts once again struggling to put the power down because of the jacks refusing to roll fluidly.

I took some videos while completing all these tests so if anyone REALLY wants to see them I can put them on youtube when I have time.

As a result of these tests I stand by my conclusion that the viscous coupler is a limited slip type differential that has the ability to mechanically sense the front and rear axle speed/grip and apportion up to 50% of engine torque to the wheels on those axles. Furthermore, the alltrac's 4WD system is not FWD until slip occurs it is a full time 50/50 4WD system with the ability for a limited difference in front and rear axle speeds.

Again, the reason our system is not FWD based 4WD is because of where the transmission is putting the power first.The transmission directly drives the center differential.The left side gear of the center differential drives the front differentialThe right side gear of the center differential drives the rear wheels via the transfercaseThe amount of power sent to either the front or the rear is decided by the center diff via the driving conditions mechanically sensed.The Viscous coupling connects the left side gear and right side gear of the center differential. When there is a bias of rotation on either side of the unit it will 'lock up' and put the car in full 4WD mode until equilibrium is achieved again.

athousandleaves wrote:As a result of these tests I stand by my conclusion that the viscous coupler is a limited slip type differential that has the ability to mechanically sense the front and rear axle speed/grip and apportion up to 50% of engine torque to the wheels on those axles. Furthermore, the alltrac's 4WD system is not FWD until slip occurs it is a full time 50/50 4WD system with the ability for a limited difference in front and rear axle speeds.

Again, the reason our system is not FWD based 4WD is because of where the transmission is putting the power first.The transmission directly drives the center differential.The left side gear of the center differential drives the front differentialThe right side gear of the center differential drives the rear wheels via the transfercaseThe amount of power sent to either the front or the rear is decided by the center diff via the driving conditions mechanically sensed.The Viscous coupling connects the left side gear and right side gear of the center differential. When there is a bias of rotation on either side of the unit it will 'lock up' and put the car in full 4WD mode until equilibrium is achieved again.

Looks good, I took a look at the BGB, and the vc looks like its driven first, you guys are on to something!One thing we should correct is how we are referring to the torque split. If the car is 4wd its not delivering 25% torque to each wheel. or 50 front 50 rear. Its actually 100% torque delivery to each wheel. comments?

phattyduck wrote:An Alltrac would pass the first set of tests (ramp, front or rear without power), but not the second set (only one wheel with power) because of the lack of locking F/R diffs and/or traction control.

For arguments sake would an Alltrac with front and rear torsens not be capable of climbing with one wheel grip?I feel that it would be able to as 50% of the power will be lost on the slipping front or rear axle while the other axle with one wheel having grip will receive up to 75% of the 50% of power into that one wheel depending on the bias for the torsen (approximately 37.5% of total engine power). I guess it would depend on the amount of power you need to actually move the vehicle up the ramp so power to weight is more of a concern once grip is available.

A Torsen can only multiply available grip from one side to the other. If one side is on 'ice' or is in the air, the other wheel will not get any torque (0x2.5 = 0). Well, it will get almost no torque, maybe a few ft-lbs. If the car had a a clutch type LSD front and rear or a really tight viscous LSD, it could transfer power around better, but then you get maintenance and wear issues. Add in a little bit of smart traction control (not available on the Alltrac, of course) and you could do just about anything (that's actually how the H1 Hummer is setup)

Don't confuse torque with wheel speed.

A viscous coupling unit will transfer torque based on the difference is shaft speed of the two inputs/outputs (like 50 ft.-lbs. at 25 RPM shaft speed difference), up to the torque limit of the unit.

A torsen LSD will transfer torque up to a pre-definded multiple (like 2.5x) of torque/grip to the opposite input/output.

A clutch-type LSD will transfer torque up to the limit of its torque handling capacity (a specific number, like 50 ft-lbs.) no mater what the grip of one side or the other is when there is difference in shaft speeds.

klue wrote:Looks good, I took a look at the BGB, and the vc looks like its driven first, you guys are on to something!One thing we should correct is how we are referring to the torque split. If the car is 4wd its not delivering 25% torque to each wheel. or 50 front 50 rear. Its actually 100% torque delivery to each wheel. comments?

Torque delivery is 50/50 when the front and rear axles are going the same speed (i.e. straight driving, not wheel spin).

When there is a difference in wheel speed, then the viscous coupler transfers torque to the side that has more traction (i.e. is spinning slower).

With an open front and rear diff, you can only get a max of 50% power to any given wheel (actually less, as there are losses in the viscous unit that leave some power going to the slipping axle). The open diffs are stuck at 50/50.

With an open front and a Torsen rear, you could get up to say 75% power on either rear wheel (assuming a 3:1 torque multiplier in the Torsen and a 100% transfer on the center viscous unit). The fronts can still only get 50% max each.

-Charlie

PS. That spinning front wheel on those launches is what allows the viscous coupler to transfer the power to the rear and get such a great launch...

I'm thinking that the maximum torque delivered from the output shaft will be the same torque value at each wheel. On the other hand one could assume that torque could be divided per wheel. But I don't think that's right. I imagine that if I uninstalled the prop shaft I would still get the same torque value on either of the front wheels. It's getting late and I'm tired , but 100% torque at each wheel sounds right to me. Luke, I think your results are correct.

Edit: Just wanted to add one thing about how we refer to the torque split. I don't think that referring to the torque split as 50/50 runs counter to saying that torque is equal at all wheels. Considering what phattyduck outlined regarding how much power can be sent through the open diffs I liken the 50/50 split to a ratio. Seen as 50:50 it can then be reduced to 1:1, meaning just as much goes to the rear as the front and vice versa. Of course, I am getting sleepy and that might have been a big mental belly flop.